1. Technical Field
This invention relates in general to disk drive vibration modes, and in particular to an apparatus and method for compensating for the thermal sensitivity of the fundamental vibration mode of a disk drive.
2. Description of the Related Art
Generally, a digital data storage system consists of one or more storage devices that store data on storage media such as magnetic or optical data storage disks. In magnetic disk storage systems, a storage device is called a hard disk drive (HDD), which includes one or more hard disks and an HDD controller to manage local operations concerning the disks. Hard disks are rigid platters, typically made of aluminum alloy or a mixture of glass and ceramic, covered with a magnetic coating. Typically, two or three platters are stacked vertically on a common spindle that is turned by a disk drive motor at several thousand revolutions per minute (rpm).
The only other moving part within a typical HDD is the head assembly. Within most drives, one read/write head is associated with each side of each platter and flies just above or below the platter""s surface. Each read/write head is connected to a semi-rigid arm apparatus which supports the entire head flying unit. More than one of such arms may be utilized together to form a single armature unit.
Each read/write head scans the hard disk platter surface during a xe2x80x9creadxe2x80x9d or xe2x80x9cwritexe2x80x9d operation. The head/arm assembly is moved utilizing an actuator which is often a voice coil motor (VCM). The stator of a VCM is mounted to a base plate or casting on which is mounted a spindle supporting the disks. The base casting is in turn mounted to a frame via a compliant suspension. When current is fed to the motor, the VCM develops force or torque which is substantially proportional to the applied current. The arm acceleration is therefore substantially proportional to the magnitude of the current. As the read/write head nears the desired track, a reverse polarity signal is applied to the actuator, causing the signal to act as a brake, and ideally causing the read/write head to stop directly over the desired track.
Modern HDD throughput and storage capacity have been substantially increased by improvement in actuator design which has resulted in increased precision and speed in head placement. The more precisely the actuator can place the read/write head, the greater the drive track density. The term xe2x80x9cservo bandwidthxe2x80x9d will be utilized hereinafter to denote the cross-over frequency of an open loop transfer function applied to head positioning systems. As the track densities of HDDs increase, a high servo bandwidth is required to improve the TMR performance. Mechanical resonance of the coil and carriage is one of the dominant factors that limit the servo bandwidth of a voice coil driven HDD.
The demand for increased speed and storage capacity has resulted in ever faster and more compact hard disk drive (HDD) assemblies. Modern disk drives typically have several stacked disks that spin on a shaft at speeds exceeding 10,000 rpm. The track densities on these disks are often more than 12,000 tracks per inch (tpi). As the track density of HDDs increases, a high servo bandwidth is required to improve the efficiency of read/write operations as measured by Track Measurement Registration (TMR) as well as other performance indicators.
It would therefore be desirable to provide an improved method and system for minimizing the contribution of dynamic mechanical deformation of a HDD suspension and head apparatus to the off-track position error of read/write heads in a digital recording system. If implemented, such a system would serve to increase the servo bandwidth and thus the effective track density of a HDD assembly.
The fundamental vibration mode of disk drive actuators, frequently called the xe2x80x9cbutterflyxe2x80x9d mode, is known to limit the servo bandwidth of disk drives. In the prior art, techniques exist in servo control to compensate for this mode and improve the bandwidth and, thus, the performance of the disk drive. One such technique assumes knowledge of the butterfly mode (i.e. its magnitude and frequency) and mathematically compensates for it in the controller algorithm.
The dynamic characteristics of the butterfly mode, such as frequency and damping, may be determined by several methods. The average values can be determined for a population of disk drives through direct measurement, or the parameters of each drive can be physically measured when it is manufactured so that the compensation for each drive is customized. However, the butterfly mode characteristics are known to be a strong function of temperature. Therefore, if the modal characteristics change due to a temperature change in the drive, the servo controller can no longer correctly compensate for the mode. The inability of a servo controller to adapt to thermal variations can lead to lower performance in the disk drive. Thus, an improved apparatus and method for compensating for the thermal sensitivity of the fundamental vibration mode of a disk drive is needed.
The fundamental vibration or butterfly mode sensitivity of a hard disk drive is significantly dependent on the operating temperature of the drive. The temperature of the drive is readily determined via measurement with a thermal or coil resistance sensor. The sensor provides the drive""s servo controller with the drive temperature. The current operating temperature of the disk drive, in combination with its modal sensitivity to thermal changes, is used to regularly update the servo controller. With this information, the servo controller calculates new butterfly mode characteristics as the operating temperature changes in order to actively compensate for the mode and thus increase the performance of the disk drive.